The invention concerns a closing unit for an injection moulding machine or transfer moulding press with a frame, a first tool carrier connected fast or rigid therewith, a second displaceable tool carrier and hydraulic displacing devices, which are eccentrically arranged with respect to the tool carriers and of which each displays a hollow cylinder fastened to the frame and a piston, which is displaceably guided therein and of which each is provided on the one side with a first shaft and on the other side with a second shaft, wherein the first shafts are arranged in such a manner that they are drawn into the hollow cylinders when the tool carriers are brought nearer to each other, and wherein one each of the tool shafts of the pistons is connected fast with the second tool carrier.
Injection moulding machines or transfer moulding presses for the production of moulded parts of synthetic material or lightweight metal include a closing unit with a locally fixed and a displaceable tool carrier. A respective part of the moulding tool is clamped on to each of the two tool carriers. In the production of a moulded part, the displaceable tool carrier is displaced in a first operating step against the locally fixed tool carrier until the two tool parts touch and the moulding tool is closed. In the next operating step, the moulding mass to be processed is pressed into the tool. Following the pressing-in of the moulded mass is a cooling phase, in which the moulded mass rigidifies. Subsequently, the displaceable tool carrier is again removed from the fixed tool carrier and the moulding tool is opened thereby so that the produced moulded part can be ejected.
In known closing units, the non-displaceable tool carrier is rigidly connected with several rods, the so-called press spars, which run parallel to one another and along which the displaceable tool carrier can be displaced. The latter is connected with the piston of a hydraulic cylinder.
To keep the moulding tool closed during the pressing-in operation, the piston must transmit a very great force on to the displaceable tool carrier. This force can amount to about 1 million kiloponds or more for machines of medium size. Due to the play of the movable elements and due to small deformations, cantings and jammings can be caused, through which the press spars are acted upon with large forces. These forces can become so great that the press spars are destroyed.
Closing units have now already become known, which have no fixed press spars. In such a previously known closing unit, two different kinds of hydraulic displacing devices are present. The fixed tool carrier is provided with four cylinders, which are characteristic of one kind of displacing device and in each of which a piston is displaceably guided, which at both sides has a shaft protruding out of the cylinder. One each of these shafts penetrates a guide of the displaceable tool carrier and is displaceably supported on the machine frame at its end remote from the piston. The shafts penetrating through the guides as well as the displaceable tool carrier are furthermore provided with detent means. These enable the shafts to be connected fast with the displaceable tool carrier before the beginning of the pressing-in of the moulding mass. The pistons are then acted upon with pressure so that they keep the moulding tool closed during the pressing-in of the moulding mass. The closing unit further includes two displacing devices of the other kind, the cylinders of which are connected fast with the displaceable tool carrier. The pistons of these displacing devices display a respective shaft at both sides. The free ends of these shafts are each connected fast with the end of a respective one of the shafts of the first-mentioned displacing devices. The two displacing devices of the second-mentioned kind enable the displaceable tool carrier, when the detent means have not notched in, to be displaced for the opening and closing of the tools. Four displacing devices of the first-mentioned kind are in that case so designed that they make possible only a short displacement, however for this the generation of a large force. The displacing devices of the second-mentioned kind are there-against constructed in such a manner that they make possible a large displacement with relatively small force.
This previously known closing unit has proved itself well with large injection moulding machines, in which very high forces are required. Because the detent-locking required before the pressing-in of the moulding mass as well as the releasing of the detent means required after the pressing-in requires a relatively large amount of time, not such a great number of pieces of moulded parts can be produced per unit time with the previously known closing unit. This is disadvantageous particularly with small machines, in which actually not such great forces are necessary.